The present invention relates to the technical field of treatment of subterranean formations using a fluid, in particular an acid fluid.
These treatments are notably useful in the petroleum and associated industries. They are used to increase the production of the well and/or to restore the production of the well when this production declines in time down to an unacceptable level of return.
There are a number of causes for the decline in production.
Within the framework of the invention, two main categories can be contemplated: a reduction in the permeability of the oil "reservoir", or the invasion of this reservoir by the water contained in a lower layer.
A reduction in permeability is due, among other things, to the entrainment of fines, by the flow of the oil, towards the production well. Around this well, these particles accumulate and gradually plug the natural pores in the rock. The oil can then no longer flow out satisfactorily through this well. These particles can be of various origins (type of rock, damage to the reservoir, progressive deterioration of the rock, etc.).
Their origin matters little, the invention being directed to the treatment, using an acid fluid, of the rocks containing particles that are at least partially soluble in this fluid in order to cause these particles to dissolve totally or partically, and thus make the natural pores in the rock re-open to such an extent as to obtain an acceptable measure of permeability.
The invasion by water of the oil producing zone is due to the difference in viscosity between oil and water. The latter, which is far more mobile than oil, is progressively entrained towards the perforations in the well by the very flow of oil towards the well. This phenomenon is known as "water coning". It will be seen below that the invention also enables this problem to be overcome.
All the above is perfectly familiar to a person skilled in the art, with regard both to the problems encountered and current solutions.
There is no need, either, to discuss in detail the structure of an oil well (or the like), as such a structure is also familiar to person skill in the art.